Our aim was to assess the effect of continuous positive airway pressure (CPAP) on the nocturnal evolution of peripheral chemosensitivity, renin-angiotensin-aldosterone system activity, sympathetic tone and endothelial biomarkers in obstructive sleep apnoea (OSA) patients with isolated nocturnal hypertension (INH) or day-night sustained hypertension (D-NSH).In a crossover randomised trial, 32 OSA patients newly diagnosed with hypertension and without antihypertensive treatment were randomly assigned to 12 weeks of CPAP or sham CPAP. Peripheral chemosensitivity was evaluated before and after sleep using the hypoxic withdrawal test (%Δ ).At baseline, D-NSH patients showed higher %Δ before sleep and higher levels of aldosterone and diurnal catecholamines. CPAP only reduced the nocturnal increase of %Δ in INH patients (6.9%, 95% CI 1.0-12.8%; p=0.026). CPAP-induced change from baseline in %Δ after sleep was 7.5% (95% CI 2.6-12.2%, p=0.005) in the INH group and 5.7% (95% CI 2.2-9.3%, p=0.004) in the D-NSH group. In contrast, %Δ before sleep only decreased with CPAP in the D-NSH patients (3.0%, 95% CI 0.5-5.6%; p=0.023).In conclusion, CPAP reduces the nocturnal increase of peripheral chemosensitivity experienced by INH patients and corrects the high daytime sensitivity of patients with D-NSH. Differences in response to CPAP between these patients can help better understand the mechanisms of perpetuation of hypertension in sleep apnoea.